Gas recirculation method and apparatus for superheat control with cold air fan static control



May 17, 1960 L.. G. 'rRoUTMAN 2,936,742

`GAS RECIRCULATION METHOD AND APPARATUS FOR SUPERHEAT CONTROL WITH COLD AIR FAN STATIC CONTROL Filed Oct. 26, 1955 2 Sheets-Sheet 1 INVENToRs G. Troui' m an ATTORNEY May 17, 1960 L G. TROUTMAN GAS RECIRCULATION M'ETHOD AND APPARATUS FOR SUPERHEAT CONTROL WITH COLD AIR FAN STATIC CONTROL Filed Oct. 26, 1955 2 Sheets-Sheet 2 ATTORNEY United SwtesoPatem O GAS RECRCULATION METHOD AND APP- RATUS FOR SUPERHEAT CONTROL WITH COLD AIR FAN STATIC CONTROL Louis G. Troutman, Madison, NJ., assignor to The Babcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Application October 26, 1955, Serial No. 542,925 9 Claims. v(Cl. 122-479) This invention relates to apparatus forand a methodt of, generating .and superheating high pressure steam, wherein the temperature of the superheated steam is maintained at a predetermined value by gas recirculation.

The invention is exemplified in a steam generating and superheating unit involving a furnace, the walls of which include steam generating tubes in which at least a predominant' portion of the total generated steam is generated as a result of radiant heat transmission from the furnace gases generated lby a coal burning means operating at temperatures above the fusion temperature of the combustible in the coal. the furnace over a convection superheater, .and a recirculated gas system withdraws lower temperature gases from a position in the vgas stream beyond the superheater and introduces the gases into the furnace in increasing volume percentage-wise as the steam generating rate decreases. In the operation of such a unit, fused or semifused particles of slag, ,from the fuel, m-ay accumulate upon the screen tubes and diiferent sections ofthe convection heating means such as the superheater, and su'ch accumulations may increase to such an extent that thesudden increase in draft loss under conditions wherein the load suddenly changes from, for example, 50 percent load to 100 percent load when, at the same time, the amount of recirculatedgas flow called for by the control system of the unit, approacheszero as the load approaches 100 percent load. The draft loss, generally speaking, varies as the'square of the change in load, and under these circumstances that would mean that the draft loss .is increased four times. This situation is apt to cause a reversal of flow of the recirculated gas through vthe fan and likewise increase the liability of ruining the fan in the manner above indicated. This invention involves a method of, and means for, preventing the above indicated type of damage to the yfan without increasing the size of the fan or increasing `the power requirements for operating the fan. According tothe invention, when the recirculated gas ilow to the furnace decreases to such an extent that the recirculated gas fan outlet static pressure approaches, to a predetermined degree, the gas pressure within the furnace, 'combustion supporting air is caused to ow to the fan inlet, this ow ofv air being controlled in such a manner that the minimum amount vof fluid handled by the fan wheel doesnot drop below thatrequired for the maximum static pressure peak of the fan. According to one embodiment of the invention a maxi-i mum flow of such air tothe fan' would occur at maximum 1 boiler load when the flow Vof recirculated gases into the furnace is zero, andino air would ow to the fan at boilerV loads. requiring recirculated gas iiow in excess of that delivered at fan static peak.VV The ow of air Vfrom the forced drafti fanoutletthrough the recirculated gas The heating gases pass from fan may, within the scope of the invention, be controlled manually or automatically.

The invention also involves a method of obtaining the above indicated results, wherein the fan 'static pressure is increased at low loads by the introduction of cool air or gas into the fan inlet, thus increasing thestatic of the fan.' According to this method of the Iinvention the cool air or gas tlow into the fan inlet is modulated to control the' temperature as required in order to give predetermined' Statics at lowl flow rates. of using lower temperature air or gas dilution for temperature control is that the static remains high, whereas the motor horsepower falls oif considerably.

determined value, and it also provides means for protecting the fan by insulating the fan from gas or aircon nection with the furnace when the pressure differential between the fan outlet and the furnace falls below a predetermined amount, for example 11/2" of Water. ln the practice of the invention pressure connections are added to the fan outlet and thefurnace inlet to measure the` from such variables as steam temperature `and rate ofV vapor generation. As the pressure differential decreases and 'approaches 11/2 of Water, -a damper is automatically opened to permit cold air from the forced draft fan to` enter the recirculatingrfan inlet. The amount of this opening Vis adjusted asrequired to allow sucientcold air j to enter and 'lower the total temperature of the recirculating gas, thus ymaintaining a ll/z pressure differential Vbetween the recirculating fan outlet and the furnace inlet. The automatic system preferably operates so that, if the dilferential pressure continues to fall, which would bey asign that the furnace resistance is increasing, and the' cold air damper reaches its wide open position so that no further margin may be gained then the system operatesl to render the fan motor inoperative. At the same time inlet and outlet dampers on the inlet and outlet sides of the fan are closed and cold air dampers upstream and downstream of the inlet and outlet dampers are opened so that the fan is blocked olf or insulated from high tem`l reversal of flow of hot gases toward the fan, the motor is' automatically stopped and the fanis automatically insu In 'addition to these fea' tures of the automatic system an alarm isalso providedV lated as previously described.

in this circuit which warns the operator that the furnace` resistance'is dangerouslyV high when the pressureditfer-j ential approaches ll/z" water.

The invention will be concisely set forth in the claims, but for a complete understanding of the invention, its uses f and advantages, recourse should be had to the following description which. refers to a preferred embodiment illus-i trated in the accompanying drawings.

Of the drawings:

A A Fig.v l is a diagrammatic representation of a gas recircu .I lation system embodying the invention and associated with a vapor generating and superheating unit; and

Fig. 2 is4 a vertical sectional view of the vapor. generat-` gas recirculation system of Fig. l is associated.

1n Pig. 1 there is -diagrammaticauy iuitrafeda cyubn One of the main advantages furnace 20 burning a fuel of such characteristics that there would be a tendency to foul the associated heat absorbing surfaces. 'Ihe furnace discharges high temperature furnace gases into a primary furnace chamber 22, from the lower part of'which the gases flow upwardly as indicated at 24 into a vertically elongated secondary furnace chamber 25,V the walls'of which include steam generating tubes. This secondary furnace chamber, the succeeding convection section including the steam superheater, and the perltinent recirculated gas system for control of superheated steam temperature over a wide range of rate of vapor generation are constructed and arranged in the manner shown and described in the pending patent application Serial No. 278,872, filed on March 27, 1952, by I. H. Koch,.now Patent No. 2,884,909 and belonging to the same assignee. As disclosed in said application and illustrated in Fig. 2, the products of combustion pass upwardly Vthrough the secondary furnace chamber 25 and over a secondary superheater 27 to a downow convection gas pass 29, then successively contact a primary superheater 31 and an eiconomizer 33, and then flow into a duct 35 leading to an airheater 37. In communication with the duct 35 is the inlet of a duct 28 of a heating gas recirculation system.

Y In the illustrative steam generating and Vsuperheating unit of Fig. 2 the elements of the superheater 27 are arranged as pendent superheater platens disposed Within the vertically elongated secondary furnace chamber 25 receiving the combustion products from the cyclone furnace, the latter being an example of fuel burning means. Parts of the pendent superheater platens are disposed in a part of the secondary furnace chamber receiving combustion products including high temperature heating gases with particles of fused slag suspended therein. In the illustrative unit there is a recirculated gas system including a fan and duct work having its inlet downsteam in a gapsflow sense from the superheater, and having its gas outlet in communication with the upper part of the primary furnace chamber 22. associated controls operate to maintain the temperature of the superheated steam ata predetermined value over a wide load range, and to accomplish this, the flow of recirculated gas to the primary furnace chamber is increased ask the load decreases, in a lower part of the load range. The recirculated gas system also functions to decrease ythe ratio of furnace absorbed heat to the total heat absorbed in superheating the vapor, as the steam demand decreases, this action also involving an increase in a proportion of the total heat absorbed by the superheater.

Referringfurther to the system shown in Fig. l as being a part of such a unit as that shown in the above identified pending patent application, the recirculated gas system includes a recirculating gas fan 26, preferably op-A erated by an appropriate electric motor and receiving partially cooled heating gases through the duct 28, the inlet of which is in communication with the gas flow of the unit at a position beyond the superheater. The outlet of the fan 26 is connected by the duct work components 30 and 32 to an outlet 34 communicating with the primary furnace chamber 22.

As the rate of steam demand decreases toward a low load value, and as the rate of tiring of the cyclone furnace 20 is decreased the dampers 36, disposed in the duct work component 30 are open and controlled or modulated to increase the recirculated gas flow through the opening 34 into the primary furnace chamber 22. The dampers 36 may be operated throughout a Wide load range by appropriate control mechanism including a damper operator 38 appropriately connected to the dampers 36 by linkage 40. The damper operator 38 is intended to respond to changes in pneumatic loading pressure in the control line 42 and the connected control line 44, leading,

from a controller 46 in which, or connected with ywhich there are suitable and known control devices for varying the loading pressure in the lines 42 and 44 in response to the conjoint influences of a plurality of variables such This gas recirculation system vand itsn 4 as load or rate of vapor generation, and superheated steam temperature. 'Y

The controls above referred to may be considered as the normal operative controls for maintaining inal steam temperature substantially at a predetermined value and, inr practice, these controls may be such as those shown. and described in the pending application to lPaul S. Dickey, Serial No. 260,357 and tiled on December 7, 1951, or in the pending patent application of Charles S. Smith, Serial No. 199,406 tiled on December 6, 1950, now abandoned, or appropriate gas recirculation control components such as those indicated in the patent application of Durham and Weaver, Serial No. 322,646 'tiled on lNovember 26, 1952. All of these `applications are assigned to the Babcock & Wilcox Company and/or its wholly owned subsidiary, The B ailey Meter Company.

l In general,V the pertinent normal operative controls of the flow of'recirculated gas operate to increase the flow of recirculated gas as the vapor generating load decreases toward a predetermined low load, with a minimum flow of recirculated gas at a relatively high load value.

. The type of unit with which the pertinent controls of recirculated gas iiow are associated is one in which the fuel burning means such as the cyclone furnace 20 operates under a positive,4or superatmospheric pressure created by such means as the forced draft fan 50 the outlet of which includes duct work having air flow control dampers 10'1 and duct work 52 leading to the air inlet of'an air heater from which the air flows by appropriate duct work to the arinlet of the fuel burning means 20. Thus there normally is a relatively high gas pressure atthe position of the recirculated gas outlet 34 into the primary furnace chamber 22, which gas pressure must be overcome by the operation of the recirculated gas fan 26 in order for the recirculated gas to flow from the fan into the primary furnace chamber. In practice, it has been determined that there must be a pressure differential between the position H, or theoutlet 34 of the recirculated gas system into the primary Afurnace chamber, and the positionl adjacent the outlet of the fany 26 of a value of the order of 11/2 of water, and to effectively utilize the actual pressure differential between these points the pressure differential controller F, or 54 has opposite sides of its diaphragm connected by the lines 56 and 5S to the outlet side of the fan 26, and to the upper part of the primary furnace chamber, respectively, at positions I and H, respectively. This pertinent pressure differential may be decreased by an increase in draft loss through the unit occasioned by abnormal slagging of the screen and various convection sections, or by a sudden increase in the rate of vapor generation. When due to such conditions, the pressure differential at F is decreased below the predetermined value, there is apt to be a reverse ilow of the high temperature heating gases fromV the upper part of the pri mary furnace chamber 22 and through the duct components 32 and 30, and through kthe fan 26. Such a reverse flow of gases would ruin the fan within a matter of minutes to cause great economic or personal injury loss not only to operators of the electric generating station with which the steam generating unit is associated but also to the users of the power developed thereby. Such a contingency is prevented by the present invention. As disclosed in the copending application of H. G. Stallkamp, Serial No. 542,926, filed October 26, fl955, when the differential pressure between the points l and H, measured by the differential pressure controller S4, approaches or drops below 11/2" of water, which means that the recirculatedgas flow is approaching a dangerous condition which might' lead to reverse gas flow, the associated controls so change the pneumatic loading pressure inthe line components 60-62 that the alarm M is sounded and, simultaneously as a result'of the signaling of the relay J through the change in loading pressure in said line componentsopens'the damperv A through the agencyof thedamper 'operator A', suiciently to permit such flow of cold air through the Vduct 102 and through the communicating duct 67 that the pressure differential between H and I is restored to a value well above 1%" of water. Simultaneously with vthe opening of the damper A, the damper D may be opened by the action of the damper operator D in response to the same loading influence in the line components 60.62, as continued through the line component 70, the damper D being likewise closed after the pressure differential between thepoints H and I has been restored to a value well above 11/2. i

-If the differential pressure between the positions'H and I continues to fallvbelow V11/2." ofvwater while one or b oth'of dampers A and D1 arewide open then the pressure switch K picks upthe `influence, or impulse throughthe lines 60 and 74 from the relay l; and through the lines 72 and 74, turns off the fanf,motor switch 76; closes the dampers E through Ythe intermediacy of the damperroperator 38 and its connections; closes the dampers C and opens the dampers B through the agency of the loading line 80, the controlcomponents ,82 and 84, the line 86, the damper operator'88 and its v connections 90--92; `to insulate the fan 26 andA permit athe relatively high pressure cold air from ,the outlet of :the forced draft fan 50 to flow through the duct com- ;ponents 100, l102 and '32 directly into the primary furnace chamber through the opening 34.

As anr additional safety factor, -if other controls fail, Jthe temperature switch L, having the element 110 re- :sponsive t'o gas temperatures withinvthe opening 34. or the upper part of the' primary furnace chamberioperzates, upon a dangerous rise in temperature, through the agency of the loading line 112 to stop the fan motor, close dampers C and E and open the dampers B through their connections which are illustrated. t

. The dampers which are shown in the drawing are preferably arranged so that they may be alternatively, manually or automatically operated through the use of selector switches indicated in one of the above identified pending applications, and thus,' for the-isolation of the recirculatingv gas -fan from high temperature furnace gases other than that above referred to the operator may manually` open the dampers A and` the dampers B to permit cold air to ow to the primary furnace and to the inlet side of the fan, and to close the dampers C and D, thus providing for bodies of cold air on both inlet and voutlet sides of the fan, after the fan motor is stopped. t Whereas the invention has been described above with reference to a steam generating and superheating unit having a'recirculated gasl flow system functioning to maintain superheat at a predetermined temperature, 'and in doing so, to rincrease the recirculated gasflow from a condition in which there is no flow of recirculated gas at top load or control load (bythe expression-top load, the top load on therecirculated gassystem Vis referred to rather than top loadlon the boiler) it is to be recognized that the invention should be considered asof such a scope as to cover the operation of a steam generating and superheating unit in which there may be appreciable ow of recirculated gas at top load on the boiler while that ow of recirculated gas may increase as indicated 6 appreciated that the invention is not necessarily limit to all of the details Vof that embodiment. The invention is, rather, to be taken ,as of a scope commensurate with the scope of the sub-joined claims.

What is claimed is:

l. In a steam generating and superheating unit of the water tube type, wall means including steam generating tubes and dening a furnace chamber, means normally supplying said chamber with high temperature heating gases, said last named means including a fuel burner. andfaforced draft fan supplying airto the burner, a convection steam superheatersubject to the gas ow from the furnace chamber, a recirculated gas system including a fan and connected ductwork normally withdrawing a percentage of the heating gases after loss of heat therefrom in the superheating and introducing the withdrawn gases into the furnace chamber,. means to detect a change in gas pressure differential between the furnace chamber and the recirculated gas f an outlet, andA means rendered effective when the pressure differential between the furnace chamber and the recirculated gas fan outlet falls below a predetermined value to provide for air flow from the forced draft fan through the re` circulated gas fan and thereby restore said pressure dif-v ferential to a value at least as high as said predetermined value, said last named means including dampered ductwork communicating with the outlet side of the forced draft fan and the inlet side of the recirculated gas fan."

2. In a steam generating and superheating unit of the water tube type, .wall means including steam generating 'tubesanddening a furnace chamber, means rnormally supplying said chamber with high temperature heating gases, a convection steam superheater subject to thegas ilow Vfrom the furnace chamber, a recirculated gas system including a fan and connected ductwork normally withdrawing a percentage of the heating gases after loss of heat therefrom in the superheating and introducing the lwithdrawn gases into the furnace chamber, means to temperature air to yeffect such increase in flow of the gas and air mixture. l A

- 3.- In a steam` generating and superheating unit of the water tube type, wall means including steam generating tubes and defining a furnace chamber, means normally supplying said'chamber with high temperature heating gases, a convectionsteam superheater subject to the gas flow from the furnace chamber, a recirculated gas system abovey as the rate ofl vapor generation approaches a*y minimum.: ;It is also to be recognized that the invention is also applicable to the operation of v'steam' generating and superheating units in which the introduced.v gases of -lower temperature function in a gas temperingl manner` Although the invention' has been described with reference to a preferred embodiment thereof,it is to l be including a fan and connected ductwork normally withstandingl a percentage of the heating gases after loss of heat therefrom in the superheating and introducing the withdrawn gases into the furnace chamber, fuel burning means firing the furnace chamber, an air heater, means supplying combustion supporting air to the air heater for subsequent flovf to the fuel .burning means, said last named vmeans including a forced draft fan, ductwork connectingthe outlet side of the forced draft fan ahead of the-air heater and the inlet ductwork of the recirculatedk gas system, means to detect a change in gas pressure-differential between the furnace chamber and the recirculated gas fan outlet, and means rendered effective whenV the pressure dilerential between the furnace-chamber and the recirculated gas fan outlet falls below a predetermined value to increase the ilow of gas and air mixture through the recirculated gas fan and thereby restore said pressure differential to a value 'at least` as highas said predetermined value, said lastcnamed means '4. fn a vapor generating and superheating unit, wallV means including vapor generating tubes normally receiving heat predominantly by radiant heat transfer from high temperature furnace gases within a furnace chamber formed by the wall means, fuel burning means in the forrn of a cyclone furnace supplying the furnace chamber with high temperature heating gases, a forced draft fau and associated ductwork normally supplying the cyclonefurnace with ksuper-atmospheric pressure combu'stion supporting air, .a convection superheater including spaced tubes internally receiving the steam generated in said generating tubes and subject externally to the flow of said heating gases after loss of heat therefrom in the vapor generating zone, a recirculated gas system vsystem outlet falls below a predetermined value to provide for sufficient flow of cold air to the fan inlet ductwork of the recirculated gas fan to establish said presvalue. t t

l5. In a high pressure steam generating and steam superheating unit, wall means including steam generating tubes and defining a furnace chamber normallyreceiving high temperature heating gases from which heat is predominantly radiantly transmitted to water within the tubes to generate steam, means including a fuel burner and a forced draft fan for `supplying the furnace chamber with high temperature furnace gases, said last Vnamed means includingforced draft fan ductwork communicatingtwi'th the fuel burner, a superheater subject to flow of the gases from the furnace chamber and `connected to the outlets of the steam generating tubes so as to superheat the generated `steam predominantly by convection heat transfer, a gas recirculatiton system including a fan. and fan inlet duct work communicating with the ow of heating gases downstream of the superheater and having fan outlet ductwork communicating with the furnace chamber, means rendered effective when the pressure differential between the furnace chamber and the recirculated gas fan voutlet falls below a predetermined value to provide air flow from the forced draft fan through the recirculated gas fan and thereby restore said pressure differential to a value at least as high as said predetermined value, said last named means including an air duct having its inlet communicating with air ow from the forced draft fan and having its outlet communicating with the gas inlet side of the recirculated gas fan, and air ow control means in said air duct for permitting air ow into the recirculated gas fan inlet ductw'ork. t Y

6. In avapor generating and superheating unit having a fluid cooled furnace, means supplying heating gases to the furnace, a superheater subject to the heating gas flow after loss of heat therefrom in vapor generation, and a recirculated gas system including a fanlhaving inlet and outlet ductwork associated therewith, the method of operation which comprises withdrawing heating gases through said inlet ductwork from a position downstream gas-wise of the superheater and discharging the withdrawn Dgases through said fan outlet ductwork into said furnace by establishing a pressure differential between the lgasesin the furnace and in the gas space at the outlet side of the gas fan, varying the percentage of the .sure differential at a Value above said predetermined heating `gases so withdrawn, and increasing the static pressure jof the fan fin response to a decrease in said pressure differential below a predetermined value by introducing-a gaseous'uid into the 'inlet of said fan at a temperature .considerably less than the temperature of the gases .flowing through said fan inlet ductwork to the gasfan to restore said pressure differential 'to a value at least as high as said predetermined value.

7.4 lIn a vapor generating andsuperheating unit, wall's including fluid cooled tubes defining arfurnace normally receiving high temperature heating gases, a superheater subject to the flow of gases from said furnace and connected forseries flow of duid from ,said tubes, a vgas recirculation system including a gas fau, fan inlet ductwork communicating with the ow of Yheating gases downstream gas-wisetofsaid superheater, and fan outlet ductworki communicating .with said furnace, and means rendered effective when the pressure differential between the furnace and the gas fan outlet falls below a predetermined value to provide ilowof a gaseous fluid into the inlet of said gas fan at a temperature considerably less than the temperature of the gases flowing through said fan inlet ductwork to the gas fan and thereby restore said fpressure differential to a value at least as high as said predetermined value. 8. `In fa vapor generating :and superheating unit, walls including fluid cooled tubes defining a furnace normally receiving high temperature heating gases, a superheater subject tothe ow of gases from said furnace and con` nected for series flow of fluid from said tubes, a gas recirculation system including a gas fan, fan inlet ductwork communicating with the ow of heating gases downstream gas-wise of said'superheater, and fan outlet ductwork communicating with said furnace, and `means rendered effective when the pressure differential between the furnace and the gas fan outletfalls below a predetermined value to provide flow of air into the inlet of said gas fan 'at a temperature considerably less than the temperature of the gases flowing through said fan inlet duct- Workrto the gas fan and thereby vrestore lsaid pressure differential to a value at least as high'as said predetermined value, said last named meansV including a forced draft fan and a vduct communicating with the outlet side ofthe forced draft `fan and the gas fan inlet ductwork and thereby restore said pressure differential to va value atleast as high as said predetermined value.

9. In a vapor generating and superheating unit, walls including fluid cooled tubes defining a furnace normally receiving high temperature Vheating gases, a superheater subject to the flow of .gases from-said furnace and connected for series flow of uid fromsaid tubes, va vgas recirculationsystem including a gas fan, fan inlet Aductwork communicating with the -ow of heating gases downstream gas-wise of said superheater, and fan outlet ductwork'fcomrnunicating with saidffurnace, means-for introducing air into the inlet of said gas fan at a temperature considerably less than the temperature of the gases fiowing through said fan inlet ductwork to the 'gas fan, and'means for isolating saidV gas fan from the heating` gas flow and for cooling the inlet and outlet sides of the gas fan including dampers in said fan inlet and outlet ductwork, a forced draft fan, land ductwork communicating with the outlet side of the forced draft vfan, the gasY fan inlet ductworktupsteam gas-wise ofthe dampers therein; Vand the gas fan outlet ductwork down stream gas-wise ofthe dampers therein.

'Luty e July 13, 1937 2,229,643 DeBaufre n e Ian. 28, 1941 p 2,730,971' BirknerV Y Y l Jani?, 1956 Y OTHER REFERENCES t B:& sW, Bulletin G74 of r 1954 page 2.4,-,

UNITED STATES PATENT 0F F ICE CERTIFICATE OF 'CORRECTION Patent No o. `2.936g742 May 1731*v 1960 Louis G, Troutman It is herez'f certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 8, line 44(l after ."ductwork insert a period.; lines 45 and loY strike out and thereby restore saci pressure n differential to a value at least as high as said `loredeternfuned value-"` l Signed and sealed this 11th day of @pril 1961 (SEAL) Attest:

ERNEST .W 'SQ/DER ARTHUR W. CROCKER AttesrtinguOc-er y Acting Commissioner of Patents 

